Conductive paste

ABSTRACT

The present invention provides an electroconductive paste that can contain a high proportion of an electroconductive powder, has excellent electroconductivity reliability and migration resistance, has a highly competitive price due to a reduced amount of silver plating, and is suitable for use in solder electrode formation, an electroconductive adhesive, etc. The electroconductive paste of the present invention comprises a binder and an electroconductive powder containing 80 to 97 wt % of a substantially spherical silver-coated copper powder in which the surface of a copper powder is coated with silver and the surface thereof is further coated with 0.02 to 0.5 wt % relative to the copper powder of a fatty acid, and 3 to 20 wt % of a flat-shaped silver-coated copper powder in which the surface of a copper powder is coated with silver and the surface thereof is further coated with 0.02 to 1.2 wt % relative to the copper powder of a fatty acid.

TECHNICAL FIELD

The present invention relates to an electroconductive paste that is usedin wiring board circuit formation, shield layer formation, electroniccomponent electrode formation, solder electrode formation, anelectroconductive adhesive, etc.

BACKGROUND ART

As one method for forming an electroconductive circuit on a printedwiring board, an electroconductive powder such as gold, silver, copper,or carbon is used, and this is mixed with a binder, an organic solvent,and, as necessary, an additive, etc. to form a paste (e.g., DenshiZairyo (Electronic Materials), October 1994, pp. 42 to 46). Inparticular, in a field in which high electroconductivity is required, agold powder, a silver powder, a palladium powder, or an alloy powderthereof is generally used.

Among the above-mentioned pastes, because it has goodelectroconductivity an electroconductive paste containing a silverpowder is used for formation of a printed wiring board, a wiring layer(an electroconductive layer) of an electronic component, etc., orformation of an electric circuit or an electrode of an electroniccomponent, but when an electric field is applied thereto underconditions of high temperature and high humidity, electrodeposition ofsilver, which is called migration, occurs on the electric circuit or theelectrode, thus causing a short circuit between electrodes or betweenwiring, which is a drawback. Several countermeasures against thismigration have been taken, for example, coating the surface of aconductor with a moisture-proof coating or adding a corrosion inhibitorsuch as a nitrogen-containing compound to the electroconductive paste,but none thereof had a sufficient effect. Although migration resistancecan be improved by use of a silver-palladium alloy powder instead of thesilver powder, since silver and palladium are expensive, thesilver-palladium alloy powder is also expensive, which is a drawback.

Furthermore, in order to obtain a conductor having favorable resistance,it is necessary to increase the amount of silver powder added, and sincethe silver powder is expensive, the electroconductive paste iscorrespondingly expensive, which is a drawback. Use of a silver-coatedcopper powder can improve the migration and also give an inexpensiveelectroconductive paste. However, when the surface of a copper powder iscoated uniformly and thickly with silver, the effect in improving themigration is not sufficient. Moreover, a coating obtained from theelectroconductive paste cannot be subjected to direct soldering, whichis a drawback. Furthermore, when the electroconductive paste employing asilver powder is subjected to soldering, silver erosion occurs and agood joint cannot be obtained, which is a drawback.

On the other hand, a copper powder is used in some cases instead of thesilver powder. However, after an electroconductive paste employing acopper powder is heated and cured, the copper is very susceptible tooxidation, and the copper powder reacts with oxygen contained in air orthe binder, thus forming an oxide film on the surface thereof andthereby greatly degrading the electroconductivity. In order to overcomethis, a copper paste having its electroconductivity stabilized by addingvarious types of reducing agent so as to prevent the oxidation of thesurface of the copper powder has been disclosed, but theelectroconductivity and the stability of the electroconductivity thereofare inferior to those of the silver paste, and the resistance thereofincreases in a high temperature, high humidity test, etc., which is adrawback.

Furthermore, unless the content of the copper powder in theelectroconductive paste is high, stable electroconductivity cannot beobtained. However, when the content of the copper powder is high, theadhesion might be degraded or the storage stability might be poor, whichis a drawback. Moreover, the coating of the copper paste thus obtainedcannot, with a conventional copper paste, be directly soldered onto,which is also a drawback.

Conventionally, when a known electroconductive paste is used as anadhesive, since electroconductive powders are expensive, theelectroconductive paste is also expensive compared with a solder paste,which is a drawback. There is therefore a desire for anelectroconductive adhesive that has a more reliable electroconductivitythan that of the copper paste, has better migration resistance than thatof the silver paste, and gives excellent workability for solder pasteand for drying/curing.

Furthermore, since conventionally known electroconductive pastes cannothave solder directly applied thereon, a coating of the electroconductivepaste is subjected to an activation treatment before carrying outelectroless plating, or copper electroplating is carried out in aplating solution using the coating as a cathode and the copper coatingobtained by plating is subsequently subjected to soldering. However,unless the bonding between the coating and the copper plating isreliable, it is not practical. Therefore, if an electroconductive pastethat can have solder applied thereon without carrying out electrolessplating or electroplating is developed, the circuit formation step canbe greatly shortened, which is a major advantage.

A solder is easily joined to a metal but does not join to a binder. Whensoldering is carried out, a coating is ideally formed from anelectroconductive powder alone, which is then subjected to soldering,but there is the problem that no coating can be formed from anelectroconductive powder alone without a binder.

A binder is therefore used to form an electroconductive paste. However,there is a restriction on the amount of binder in order to achievereliability and workability of coating formation; for example, if theproportion of the binder is high, the electroconductive powder, which isa metal, is covered with the binder, there can be no areas where thesolder and the electroconductive powder are in contact with each other,the solder cannot therefore adhere, and the electroconductivity isdegraded, which are drawbacks.

In order to form an electroconductive paste to which a solder canadhere, the composition should be as close as possible to that of acopper foil. That is, it is ideal for the paste to have a compositionsuch that when the electroconductive powder is placed in a given space,the electroconductive powder is highly packed, and the binder occupiesonly the volume corresponding to gaps between the electroconductivepowder particles.

However, when the proportion of the electroconductive powder is madehigh as described above, the viscosity of the electroconductive pastebecomes extremely high, and it becomes difficult to prepare anelectroconductive paste, the workability when applying theelectroconductive paste is degraded, and since the amount of binder forcombining the electroconductive powder particles is small, the strengthof the coating is also degraded. Furthermore, when the paste is used asan electroconductive adhesive, since the adhesion is low, it is notsuitable for that use. Moreover, when solder joining is carried outusing the electroconductive paste, it is necessary to use anelectroconductive paste having a good balance between solderingproperties, electroconductivity, workability, strength, and cost.

When the electroconductive paste is used as a solder substitute materialfor the purpose of electroconductive adhesion, in addition toprintability, adhesion, and reliable conductivity, the workability, thatis, the paste being dried and cured in a short time, is also important.When drying and curing a solder substitute adhesive, it is preferable touse a reflow oven, which has been used by assembly manufacturers forsoldering chip components, from the viewpoint of effective utilizationof equipment. In the case of general silver pastes, there is the defectthat drying and curing at high temperature for a short time in a solderreflow oven easily causes swelling. Copper pastes also have the defectsthat curing at high temperature for a short time does not give stableelectroconductivity, and in reliability tests such as a constanttemperature, constant humidity test and a gas-phase cooling and heatingtest, the so-called open circuit, that is, loss of conduction, isobserved.

An electroconductive paste is used in a method for forming anelectroconductive layer as shown in FIG. 1 by dispersing anelectroconductive powder in a binder to form an electroconductive paste,and coating the surface of a substrate with the electroconductive pasteor filling a through hole therewith. In FIG. 1, 1 denotes theelectroconductive paste and 2 denotes a copper foil.

With regard to another means for forming an electroconductive layer in athrough hole formed in a printed wiring board, there is a method forforming an electroconductive layer by subjecting the inner wall of thethrough hole to copper plating.

When a hole-filling electroconductive paste for filling a through holeis employed connection between layers generally requires highelectroconductivity even though the hole is small, and it is thereforenecessary to pack the hole with the electroconductive paste as tightlyas possible, and embed the electroconductive paste in the hole withoutany gaps. Conventional hole-filling electroconductive pastes aretherefore required to have a high proportion of electroconductivepowder, but when the proportion of electroconductive powder is madehigh, the viscosity of the electroconductive paste increases, and thehole packing properties thereof are degraded. Conversely, when theproportion of binder is made high, the viscosity decreases and the holepacking properties are therefore improved, but the electroconductivityis degraded, which is a drawback.

As a countermeasure thereagainst, a solvent-free type ofelectroconductive paste containing no solvent and employing as a maincomponent of the binder a liquid epoxy resin is used, or anelectroconductive paste is used that contains, depending on the holesize, a small amount of solvent.

However, since the epoxy resin shrinks less due to heat curing than aphenolic resin does, it is difficult to lower the resistance of theelectroconductive paste containing the epoxy resin as a main component,which is a drawback.

Although the resistance can be reduced by increasing the proportion ofelectroconductive powder in the electroconductive paste or by using ametal powder having high electroconductivity such as silver, theresulting electroconductive paste is expensive.

On the other hand, there is an electroconductive paste containing aphenolic resin as a main component, and although this electroconductivepaste has better electroconductivity than that of the electroconductivepaste containing the epoxy resin as a main component, the viscosity ofthe electroconductive paste is high and there is a problem in terms ofthe hole packing properties.

When an electroconductive layer is formed within a through hole usingthe electroconductive paste, if the electroconductive paste used forfilling the through hole contains a large amount of solvent, voids areinevitably formed within the through hole during drying of the solvent.The voids are a drawback for a multilayer circuit board as shown in FIG.2 in which an insulating layer 5 is formed on the surface of a substrate3, an end portion of the through hole filled with the electroconductivepaste, a copper foil land 7, and part of a copper foil circuit 8, and anelectroconductive material printed circuit (hereinafter, called aprinted circuit) is further formed on the insulating layer 5 using anelectroconductive material (jumper electroconductive paste), and it isnecessary to eliminate any voids within a through hole 10, therebyimproving the connection reliability between the through hole 10, thecopper foil land 7, the copper foil circuit 8, and the printed circuit.In FIG. 2, 4 denotes an electroconductive layer, 6 denotes a jumpercircuit, and 9 denotes an overcoat layer.

When a multilayer circuit board is fabricated by making a through holeconduct by means of copper plating formed on the inner wall of thethrough hole, after the inner wall of the through hole is subjected tocopper plating, by applying cap plating over the electroconductive pastewith which the through hole is filled, the above-mentioned drawbacks canbe eliminated, but the number of steps increases and the cost alsoincreases, which is undesirable.

There is also a method in which the inner wall of a through hole issubjected to copper plating so as to form an electroconductive layer,and the cavity is filled with a resin, and this method also has thedefect that the number of steps increases and the cost is thus high.

There is also a method in which a through hole is filled with a voidlessor substantially voidless electroconductive material so as to ensureconduction of the through hole, and an insulating layer and a printedcircuit are then formed on the surface of a substrate. In this method,since the electroconductive material with which the through hole isfilled and a copper foil land portion are connected to each other viathe cross section of an end portion of the copper foil, there is thedefect that the reliability of the connection is low. In order toeliminate this defect, the above-mentioned cap plating may be carriedout, but this increases the number of steps and the cost, which isundesirable.

When a multilayer circuit board is fabricated by using a silver throughhole wiring board in which a through hole is filled with a silverelectroconductive material (silver paste) containing a solvent at 15 wt% or more and forming an insulating layer and a printed circuit on thesurface of this wiring board, a large cavity is formed within thethrough hole accompanying evaporation of the solvent, resulting in adecrease in the reliability. That is, when an ionic impurity remainswithin the void during a washing step, etc., the migration resistance isdegraded. Furthermore, in the case of the silver through hole wiringboard, the silver paste might be thickly built up on a copper foil land,and the height of this thick, built up silver paste might obstruct themounting of components.

On the other hand, there is a soldering material containing lead as amain component, and such a soldering material has been widely put intopractice for a long time because it has a comparatively low meltingpoint and good workability.

However, recently, restrictions on the use of lead have been proposedsince lead is highly toxic and the human body and the ecosystem might beeasily affected when a lead-containing effluent is treated. At present,a low melting point metal brazing material employing, as a substitutefor lead, a metal material having a comparatively low melting point suchas bismuth is being developed, but since the melting point thereof ishigher than that of lead solder, it is necessary to increase the heatresistance of the substrate material, electronic components to bemounted, etc., giving rise to the defects of technical difficulty, anincrease in cost, etc.

A multilayer lamination step employing a standard hole-fillingelectroconductive paste involves filling a hole with theelectroconductive paste, laminating a pre-dried build-up layer, andapplying heat and pressure as the main drying. It is therefore necessarythat the electroconductive paste is cured after the main drying, and itis also necessary that the electroconductivity is improved by theapplication of pressure after lamination compared with a case wherepressure is not applied.

However, the conventional hole-filling electroconductive paste employsan epoxy resin as a main component of the binder, and generally uses animidazole as a curing agent therefor, and when a substantially sphericalsilver-coated copper powder that has been subjected to a dispersiontreatment to break up aggregates and has copper exposed on the surfacethereof is used as an electroconductive powder, the curing properties ofthe electroconductive paste might be degraded, which is a drawback.

When a silver-coated copper powder that has been subjected to adispersion treatment is used, it is necessary to add a material thatdoes not form a chelating bond with copper and functions as a curingagent for an epoxy resin.

Furthermore, since the substantially spherical silver-coated copperpowder easily aggregates in a silver plating step and has a low tapdensity, if a large proportion thereof is added to the electroconductivepaste, the viscosity of the paste is undesirably increased.

Moreover, when the substantially spherical silver-coated copper powderthat has been subjected to the dispersion treatment is used, since aresol type phenolic resin forms a chelating bond with the copper, theviscosity of the electroconductive paste increases during storage, whichis a drawback.

Furthermore, when an electroconductive adhesive (electroconductivepaste) is prepared using an alkoxy group-containing resol type phenolicresin and an epoxy resin as binders, if a problem occurs in a componentthat is bonded to a printed wiring board and the bonded component isreplaced, it is necessary to heat the cured thermosetting resins to ahigh temperature so that they are in a rubbery state, and such a defectcan be eliminated by the use of a thermoplastic resin as the binder.

The present invention provides an electroconductive paste that cancontain a high proportion of an electroconductive powder, has excellentelectroconductivity reliability and migration resistance, has a highlycompetitive price due to a reduced amount of silver plating, and issuitable for use in solder electrode formation, an electroconductiveadhesive, etc.

Furthermore, the present invention provides an electroconductive pastethat has good packing properties and excellent flowability.

Moreover, the present invention provides an electroconductive paste thathas excellent shelf life, can be cured in a short time, has excellentshort-time drying and curing properties when using an infrared oven(hereinafter, called an IR oven), and is suitable for forming a wiringboard circuit, filling a hole, etc.

Furthermore, the present invention provides an electroconductive pastethat can give low viscosity and a high degree of packing, and has goodheat resistance since the epoxy equivalent is small.

Moreover, the present invention provide an electroconductive paste thathas a stable shelf life.

Furthermore, the present invention provides an electroconductive pastethat has suppressed spreading during drying after printing.

Moreover, the present invention provides an electroconductive paste thathas, in particular, excellent curing properties.

Furthermore, the present invention provides an electroconductive pastethat has an excellent shelf life and is suitable as an electroconductiveadhesive that enables adhered components to be easily removed.

Moreover, the present invention provides an electroconductive paste thathas good electroconductivity and a stable shelf life.

Furthermore, the present invention provides an electroconductive pastethat has suppressed spreading during drying after printing, andexcellent adhesion and flexibility.

Moreover, the present invention provides an electroconductive paste thathas few voids under fast curing conditions using a reflow oven, hasexcellent adhesion, electroconductivity and printability, and issuitable for mounting a semiconductor device, a passive component, etc.

DISCLOSURE OF INVENTION

The present invention relates to an electroconductive paste comprising abinder and an electroconductive powder containing 80 to 97 wt % of asubstantially spherical silver-coated copper powder in which the surfaceof a copper powder is coated with silver and the surface thereof isfurther coated with 0.02 to 0.5 wt % relative to the copper powder of afatty acid, and 3 to 20 wt % of a flat-shaped silver-coated copperpowder in which the surface of a copper powder is coated with silver andthe surface thereof is further coated with 0.02 to 1.2 wt % relative tothe copper powder of a fatty acid.

Furthermore, the present invention relates to an electroconductive pastewherein the substantially spherical silver-coated copper powder has anaverage particle size of 1 to 10 μm and a tap density of 55% to 75% as arelative value to a true density, and the surface thereof is smoothed.

Moreover, the present invention relates to an electroconductive pastewherein the binder comprises as main components an alkoxygroup-containing resol type phenolic resin and an epoxy resin togetherwith a curing agent, an additive, and a solvent therefor.

Furthermore, the present invention relates to an electroconductive pastewherein the epoxy resin has an epoxy equivalent of 130 to 330 g/eq.

Moreover, the present invention relates to an electroconductive pastewherein the alkoxy group-containing resol type phenolic resin has analkoxy group having 1 to 6 carbons.

Furthermore, the present invention relates to an electroconductive pastewherein the alkoxy group-containing resol type phenolic resin has adegree of alkoxylation of 5% to 95%.

Moreover, the present invention relates to an electroconductive pastewherein the alkoxy group-containing resol type phenolic resin has aweight-average molecular weight of 500 to 200,000.

Furthermore, the present invention relates to an electroconductive pastewherein the alkoxy group-containing resol type phenolic resin and theepoxy resin are present at an alkoxy group-containing resol typephenolic resin:epoxy resin mixing ratio of 5:95 to 60:40 as a ratio byweight.

Moreover, the present invention relates to an electroconductive pastewherein the binder comprises as main components a thermoplastic resintogether with an additive and a solvent.

Furthermore, the present invention relates to an electroconductive pastewherein the thermoplastic resin is a thermoplastic resin having asoftening temperature of 90° C. to 240° C.

Moreover, the present invention relates to an electroconductive pastewherein the thermoplastic resin is a phenoxy resin.

Furthermore, the present invention relates to an electroconductive pastewherein the binder comprises as main components an epoxy resin togetherwith a curing agent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing a state in which a through holeis connected by means of an electroconductive paste.

FIG. 2 is a cross sectional view of a conventional through hole wiringboard.

FIG. 3 is a top view showing a state in which a test pattern is formedon a polyimide film.

BEST MODE FOR CARRYING OUT THE INVENTION

The electroconductive paste of the present invention comprises a binderand an electroconductive powder containing 80 to 97 wt % of asubstantially spherical silver-coated copper powder in which the surfaceof a copper powder is coated with silver and the surface thereof isfurther coated with 0.02 to 0.5 wt % relative to the copper powder of afatty acid, and 3 to 20 wt % of a flat-shaped silver-coated copperpowder in which the surface of a copper powder is coated with silver andthe surface thereof is further coated with 0.02 to 1.2 wt % relative tothe copper powder of a fatty acid.

With regard to the substantially spherical silver-coated copper powder,the amount of silver with which the surface of the copper powder iscoated is not particularly limited, but it is preferably in the range of2.5 to 12 wt % relative to the copper powder, and more preferably in therange of 2.5 to 7.5 wt %. When the amount of silver coating exceeds 12wt %, the degree of aggregation during a silver coating step increases,the tap density tends to decrease and the cost increases. When it isless than 2.5 wt %, the degree of copper exposed increases, and thereliability of the electroconductivity tends to be degraded easily.

The average particle size of the substantially spherical silver-coatedcopper powder used in the present invention is preferably in the rangeof 1 to 10 μm from the viewpoint of handling during printing anddischarge, etc., and the cost, and is more preferably in the range of 2to 7 μm.

Furthermore, the aspect ratio of the substantially sphericalsilver-coated copper powder is preferably in the range of 1 to 1.5, andmore preferably in the range of 1 to 1.3.

On the other hand, with regard to the flat-shaped silver-coated copperpowder, the amount of silver with which the surface of the copper powderis coated is not particularly limited, but it is preferably in the rangeof 3 to 12 wt % relative to the copper powder, and more preferably inthe range of 3 to 10 wt %. When the amount of silver coating exceeds 12wt %, the cost tends to increase. When it is less than 3 wt %, thereliability of the electroconductivity tends to be low.

The average particle size of the flat-shaped silver-coated copper powderused in the present invention is preferably no more than 10 μm from theviewpoint of preventing peel-off or damage of the silver coating layeron the surface when preparing the flat-shaped silver-coated copperpowder, and is more preferably in the range of 6.5 to 9 μm.

Furthermore, the aspect ratio of the flat-shaped silver-coated copperpowder is preferably in the range of 2 to 20, and more preferably in therange of 2 to 15.

The average particle size referred to here is a value measured using alaser scattering type particle size distribution analyzer. In thepresent invention, a Mastersizer (manufactured by Malvern Instruments)is used as the analyzer.

The aspect ratio referred to in the present invention denotes the ratio(major axis/minor axis) of the major axis to the minor axis of thesubstantially spherical silver-coated copper powder. In the presentinvention, the substantially spherical silver-coated copper powderparticles are mixed well in a curable resin having low viscosity, themixture is allowed to stand so that the particles settle and at the sametime the resin is cured in that state, the cured substance thus obtainedis sectioned in the vertical direction, the shape of particles appearingin the section is inspected by an electron microscope, the majoraxis/minor axis of each particle for at least 100 particles is measured,and the average value thereof is defined as the aspect ratio.

The minor axis referred to here is the distance between two parallellines having the shortest distance among pairs of parallel lines thatare selected so as to sandwich the particle appearing in theabove-mentioned section and be tangential to the outside of theparticle. The major axis referred to here is the distance of twoparallel lines having the longest distance among pairs of parallel linesthat are orthogonal to the parallel lines defining the minor axis andare tangential to the outside of the particle. The rectangle formed bythese four lines has a size within which the particle is just contained.

Specific methods employed in the present invention will be explainedbelow.

In the present invention, a method for coating the surface of the copperpowder with silver is not particularly limited; there are methods suchas, for example, displacement plating, electroplating, and electrolessplating, and displacement plating is preferable since the adhesionbetween the copper powder and silver is strong and the running cost islow.

In the present invention, the surface of the silver-coated copper powderin which the surface of the copper powder is coated with silver isfurther coated with a fatty acid. Examples of the fatty acid used in thepresent invention include a saturated fatty acid such as stearic acid,lauric acid, capric acid, or palmitic acid, and an unsaturated fattyacid such as oleic acid, linoleic acid, linolenic acid, or sorbic acid.

The amount of fatty acid with which the surface of the silver-coatedcopper powder is coated is in the range of 0.02 to 0.5 wt % relative tothe copper powder when the shape thereof is substantially spherical,preferably in the range of 0.02 to 0.2 wt %, and more preferably in therange of 0.02 to 0.1 wt %. When it exceeds 0.5 wt %, the aggregatedsilver-coated copper powder particles can be easily dispersed, and thesilver-coated copper powder can be easily wetted by a resin solution,but since the fatty acid functions as an internal mold release agent,the adhesion is degraded. When the amount of fatty acid coating is lessthan 0.02 wt %, it is difficult to disperse the aggregated silver-coatedcopper powder particles.

When the shape of the silver-coated copper powder is flat, the amount offatty acid coating is in the range of 0.02 to 1.2 wt % relative to thecopper powder, preferably in the range of 0.08 to 1.0 wt %, and morepreferably in the range of 0.15 to 0.7 wt %. When it exceeds 1.2 wt %,although the flat-shaped silver-coated copper powder can be easilywetted by a resin solution, since the fatty acid functions as aninternal mold release agent, when it is used as an adhesive, theadhesion is degraded. When it is less than 0.02 wt %, it is difficult toprocess into a flat shape.

Coating the surface of the silver-coated copper powder with the fattyacid has the following advantages. That is, when the copper powder issubjected to silver plating, the moisture contained in the copper powderis dried in a subsequent drying step. During this step, if the moistureis dried directly, since the latent heat of evaporation of water islarge, it takes a long time to dry. However, when the moisture isreplaced in advance with a hydrophilic organic solvent such as analcohol or acetone, this organic solvent can be easily dried. Thepresent invention utilizes this effect, and by mixing the fatty acidwith the organic solvent so as to make drying easy and by setting theamount of fatty acid coating in the above-mentioned range, theaggregated silver-coated copper powder particles are easily dispersed, asubstantially spherical silver-coated copper powder having no problemwith adhesion and having a high tap density can be obtained, and aflat-shaped silver-coated copper powder that can be easily wetted by aresin solution and has no problem with adhesion can also be obtained.

In the present invention, with regard to the electroconductive powders,the above-mentioned substantially spherical silver-coated copper powderand flat-shaped silver-coated copper powder are used.

With regard to the mixing proportions of the substantially sphericalsilver-coated copper powder and the flat-shaped silver-coated copperpowder, the substantially spherical silver-coated copper powder is 80 to97 wt % and the flat-shaped silver-coated copper powder is 3 to 20 wt %,and the substantially spherical silver-coated copper powder ispreferably 85 to 97 wt % and the flat-shaped silver-coated copper powderis preferably 3 to 15 wt %. When the substantially sphericalsilver-coated copper powder is less than 80 wt % and the flat-shapedsilver-coated copper powder exceeds 20 wt %, there is the problem thatthe reliability of the electroconductivity might be degraded, and whenthe substantially spherical silver-coated copper powder exceeds 97 wt %and the flat-shaped silver-coated copper powder is less than 3 wt %,there is the problem that the reliability of the electroconductivitymight be degraded.

The substantially spherical silver-coated copper powder preferably has atap density in the range of 55 to 75% as a relative value, and morepreferably in the range of 58 to 75%. When the tap density is less than55%, since the packing density is low, if the mixing proportion of theelectroconductive powder is increased, the viscosity of theelectroconductive paste increases, whereas if the mixing proportion ofthe electroconductive powder is decreased, the electroconductivity andthe reliability tend to be insufficient. Although a silver-coated copperpowder can be obtained by subjecting a copper powder to silver plating,the surface of the silver-coated copper powder that has only been platedhas microscopic silver crystals precipitated thereon and is not smooth,and the flowability between particles also tends to be degraded.Furthermore, since there are particle boundaries between the microscopicsilver crystals, the electroconductivity of the silver plating layeritself can be low in some cases. Moreover, when silver plating iscarried out, the adhesion between the silver plating layer and thecopper powder as a core material might not be sufficient. On the otherhand, the substantially spherical silver-coated copper powder having atap density that exceeds the upper limit of 75% becomes difficult toprepare.

Furthermore, the tap density of the flat-shaped spherical silver-coatedcopper powder is preferably in the range of 27% to 50% as a relativevalue, and more preferably in the range of 30% to 45%. When the tapdensity is less than 27%, if it is used in combination with thesubstantially spherical silver-coated copper powder, since the packingdensity is lowered, the flowability tends to decrease. On the otherhand, when the tap density exceeds 50%, the shape is closer to a sphere,and the effect in improving the contact between the substantiallyspherical silver-coated copper powder particles tends to be low.

The relative value of the tap density referred to here is a valueobtained by carrying out tapping with a stroke of 25 mm 1000 times,calculating the tap density from the volume and the weight, and dividingit by the true density or the theoretical density of the particles.

Preferred examples of the binder used in the present invention includeone comprising as main components an alkoxy group-containing resol typephenolic resin and an epoxy resin, together with a curing agenttherefor, an additive, and a solvent, one comprising as main componentsa thermoplastic resin, together with an additive, and a solvent, and onecomprising as main components an epoxy resin, together with a curingagent.

An electroconductive paste employing a phenolic resin gives a higherelectroconductivity than that of an electroconductive paste employing anepoxy resin alone. This is because since the amount of shrinkage due tocuring of the phenolic resin is larger than that of the epoxy resin, thedecrease in volume of the electroconductor is large, and the contactarea and the probability of contact between the electroconductive powderparticles are large. It is essential for an electroconductive pastewhere high electroconductivity is required to contain a phenolic resin,but the viscosity of the electroconductive paste easily increases, andit is difficult to increase the mixing proportion of theelectroconductive powder; however, use of the alkoxy group-containingresol type phenolic resin can eliminate these problems.

Even when the alkoxy group-containing resol type phenolic resin is mixedwith the substantially spherical silver-coated copper powder on whichcopper is exposed, since methylol groups of the phenolic resin aremasked with alkoxy groups, it is possible to suppress the reactionbetween the copper surface and the methylol groups.

On the other hand, since the epoxy resin has excellent mechanicalproperties, heat resistance, and adhesion, it is suitable as a binder inan adhesive, etc. However, when an imidazole alone is used as a curingagent, if the curability is increased, a dark reaction at roomtemperature cannot be prevented, and the shelf life is inevitablyshortened. However, when the above-mentioned alkoxy group-containingresol type phenolic resin and an imidazole are used in combination ascuring agents for the epoxy resin, an electroconductive paste having along shelf life and excellent curing properties at around 160° C. can beobtained.

When there is a need for replacing a component adhered to a printedwiring board, an electroconductive adhesive (electroconductive paste)obtained using a thermoplastic resin has the effect of enabling theadhered component to be more easily replaced than one adhered using analkoxy group-containing resol type phenolic resin and an epoxy resin.

An electroconductive paste obtained using a epoxy resin, when it isapplied for use of mounting a semiconductor device, a passive component,etc. on a substrate, can exhibit a specific volume resistivity andadhesion because of voids are rarely formed under fast curing conditionsusing a reflow oven. Furthermore they are electroconductive pastes thathave the adhesion required for a material for connecting a semiconductordevice, a passive component, etc., they can be used in a printing stepof a production process in which a solder paste has conventionally beenused, and they have adequate characteristics as solder substitutes.

A solvent can be used in order to adjust the viscosity and control theworkability during printing, discharge, etc. When the boiling point ofthe solvent is low, the viscosity changes greatly while working, whichis undesirable, and when the boiling point thereof is too high, thedrying properties are poor and problems are caused in curing and dryingprocesses. It is preferable to use a solvent having a boiling point atatmospheric pressure of 150° C. to 250° C., and more preferably 170° C.to 240° C. Examples of the solvents that satisfy the above-mentionedrequirements include ethyl carbitol, dipropylene glycol methyl ether,dipropylene glycol ethyl ether, dipropylene glycol isopropyl methylether, dipropylene glycol isopropyl ethyl ether, tripropylene glycolmethyl ether, propylene glycol ethyl ether acetate, ethylene glycolethyl ether acetate, ethylene glycol butyl ether, diethylene glycolmethyl ether, diethylene glycol ethyl ether, 3-methyl-3-methoxybutanol,3-methyl-3-methoxy butyl ether, and butyl lactate.

The epoxy resin used in the present invention is preferably liquid atroom temperature. A crystalline epoxy resin that crystallizes at roomtemperature can also be used as long as crystallization can be preventedby mixing it with a liquid substance. The epoxy resin that is liquid atroom temperature referred to in the present invention includes, forexample, one that is solid at room temperature, but is converted into aliquid stably by mixing it with an epoxy resin that is liquid at roomtemperature. The room temperature referred to in the present inventionmeans a temperature of about 25° C.

The epoxy resin used preferably has an epoxy equivalent in the range of130 to 330 g/eq, and more preferably in the range of 160 to 250 g/eq.

A known epoxy resin such as a compound having at least two epoxy groupsin the molecule is used, and examples thereof include an aliphatic epoxyresin such as polyglycidyl ether, dihydroxynaphthalene diglycidyl ether,butanediol diglycidyl ether or neopentyl glycol diglycidyl etherobtained by reaction of bisphenol A, bisphenol AD, bisphenol F, novolac,or cresol novolac with epichlorohydrin; a heterocyclic epoxy such asdiglycidyl hydantoin; and an alicyclic epoxy resin such asvinylcyclohexene dioxide, dicyclopentanediene dioxide, or an alicyclicdiepoxy adipate.

A flexibility imparting agent is added as necessary. A known flexibilityimparting agent such as a compound having only one epoxy group in themolecule may be used. Examples thereof include ordinary epoxy resinssuch as n-butyl glycidyl ether, glycidyl versatate, styrene oxide,ethylhexyl glycidyl ether, phenyl glycidyl ether, cresyl glycidyl ether,and butylphenyl glycidyl ether.

These epoxy resins and flexibility imparting agents can be used singlyor in a combination of two or more types.

By using in combination the alkoxy group-containing resol type phenolicresin and a conventionally used curing agent, as described above, theshelf life increases, the curing properties become excellent, and thesolvent resistance of a cured electroconductive paste material improves.In particular, by the use or combined use of curing agents havingdifferent melting points and dissociation temperatures, the semi-curedstate of an electroconductive paste can be controlled, which ispreferable. With regard to the curing agents, imidazoles are preferablefrom the viewpoint of the pot life; examples of other curing agentsinclude amines such as menthenediamine, isophoronediamine,metaphenylenediamine, diaminodiphenylmethane, diaminodiphenylsulfone,and methylenedianiline, acid anhydrides such as phthalic anhydride,trimellitic anhydride, pyromellitic anhydride, succinic anhydride, andtetrahydrophthalic anhydride, and dicyandiamide; a curing agent such asa latent amine curing agent may be used in combination as necessary; anda compound such as tertiary amine, triphenylphosphine, ortetraphenylphosphenyl borate may be added.

The content of such a curing agent is preferably in the range of 0.1 to20 parts by weight relative to 100 parts by weight of the epoxy resinfrom the viewpoint of the glass transition temperature (Tg) of the curedelectroconductive paste material, and is more preferably in the range of1 to 10 parts by weight.

The number of carbons of the alkoxy group of the alkoxy group-containingresol type phenolic resin is preferably 1 to 6 from the viewpoint of theviscosity and the electroconductivity when used as a binder of theelectroconductive paste, and is more preferably 2 to 4.

Furthermore, the degree of alkoxylation of the resol type phenolicresin, that is, the percentage of alkoxylated methylol groups relativeto the total amount of methylol groups, is preferably in the range of 5%to 95% from the viewpoint of the viscosity, the electroconductivity, andthe reliability of the electroconductive paste, and is more preferablyin the range of 10% to 85%.

Moreover, the number of alkoxy groups in the alkoxy group-containingresol type phenolic resin is preferably in the range of 0.1 to 2 perbenzene ring, more preferably in the range of 0.3 to 1.5, and yet morepreferably in the range of 0.5 to 1.2.

The degree of alkoxylation or the number of alkoxy groups can bemeasured by a nuclear magnetic resonance spectroscopic method(hereinafter, called an NMR method).

The weight-average molecular weight of the alkoxy group-containing resoltype phenolic resin of the present invention is preferably in the rangeof 500 to 200,000 from the viewpoint of the viscosity of theelectroconductive paste, the shelf life, the curing properties of theelectroconductive paste, the electroconductivity, the adhesion, thetoughness, etc., and is more preferably in the range of 500 to 120,000.

The weight-average molecular weight can be measured by a gel permeationchromatographic method and calculated using polystyrene as a standard.

The mixing ratio of the alkoxy group-containing resol type phenolicresin and the epoxy resin is preferably 5:95 to 60:40 as an alkoxygroup-containing resol type phenolic resin:epoxy resin ratio by weight,and more preferably 10:90 to 40:60. When the proportion of the alkoxygroup-containing resol type phenolic resin is less than theabove-mentioned range, the function as a curing agent is degraded, andthe electroconductivity tends to be poor. When it exceeds theabove-mentioned range, although the electroconductivity of theelectroconductive paste is high, the balance between the adhesion, thetoughness, the viscosity, etc. tends to be poor.

The thermoplastic resin preferably has a softening temperature of 90° C.to 240° C., and more preferably 130° C. to 200° C. Examples thereofinclude a phenoxy resin, a thermoplastic polyester resin, and apolystyrene resin, and it is preferable to use, among these, a phenoxyresin having a softening temperature of 90° C. to 240° C. sinceexcellent mechanical strength, heat resistance, and adhesion can beobtained. When mixed with the substantially spherical silver-coatedcopper powder on which copper is exposed, the thermoplastic resinenables a reaction between the copper surface and a functional group tobe suppressed. Furthermore, use of the thermoplastic resin can providean electroconductive paste that has a long shelf life and can be driedreadily at around 100° C. to 160° C.

The binder used in the present invention can be obtained by adding asnecessary to the above-mentioned materials a coupling agent (additive)such as a silane coupling agent, a titanate coupling agent, an aluminatecoupling agent, etc., a thixotropic agent, an antifoaming agent, apowder surface treating agent, a precipitation inhibitor, etc., andmixing uniformly. The contents of the coupling agent, the thixotropicagent, the antifoaming agent, the powder surface treating agent, theprecipitation inhibitor, etc., which are added as necessary, arepreferably in the range of 0.01 to 1 wt % relative to theelectroconductive paste, and more preferably in the range of 0.03 to 0.5wt %.

The electroconductive paste of the present invention can be obtained byuniformly mixing and dispersing the binder and the electroconductivepowder, together with the coupling agent, thixotropic agent, antifoamingagent, powder surface treating agent, precipitation inhibitor, solvent,etc., which are added as necessary, in a mortar and pestle machine, akneader, a three roll mill, etc.

In the present invention, the mixing ratio of the binder and theelectroconductive powder is preferably 3:97 to 40:60 as abinder:electroconductive powder ratio by weight, and more preferably10:90 to 25:75. When the proportion of the binder is less than the abovementioned range, the viscosity tends to be high. When it exceeds theabove mentioned range, the electroconductivity tends to poor.

The electroconductive paste of the present invention has excellentelectroconductivity reliability and migration resistance and is suitablefor use in solder electrode formation, an electroconductive adhesive,etc.

Furthermore, the electroconductive paste of the present invention hasgood packing properties and excellent flowability.

Moreover, the electroconductive paste of the present invention hasexcellent shelf life has excellent short-time drying and curingproperties when using an IR oven.

Furthermore, the electroconductive paste of the present invention cangive low viscosity and a high degree of packing, and has good heatresistance since the epoxy equivalent is small.

Moreover, the electroconductive of the present invention paste that hasa stable shelf life.

Furthermore, the electroconductive paste of the present invention hassuppressed spreading during drying after printing.

Moreover, the electroconductive paste of the present invention hasexcellent curing properties.

Furthermore, the electroconductive paste of the present invention has anexcellent shelf life and is suitable as an electroconductive adhesivethat enables adhered components to be easily removed.

Moreover, the electroconductive paste of the present invention has goodelectroconductivity and a stable shelf life.

Furthermore, the electroconductive paste of the present invention hassuppressed spreading during drying after printing, and excellentadhesion and flexibility.

Moreover, the electroconductive paste of the present invention has fewvoids under fast curing conditions using a reflow oven, has excellentadhesion, electroconductivity and printability, and is suitable formounting a semiconductor device, a passive component, etc.

EXAMPLES

The present invention is explained below by means of Examples.

Example 1

38 parts by weight of an alkoxy group-containing resol type phenolicresin (a trial product of the applicant, number of carbons of the alkoxygroup 4, degree of alkoxylation 65%, weight-average molecular weight1,200), 57 parts by weight of a bisphenol F type epoxy resin having anepoxy equivalent of 170 g/eq (product name Epomik R110, manufactured byMitsui Chemical Co., Ltd.), and 5 parts by weight of2-phenyl-4-methylimidazole (product name Curezol 2P4MZ, manufactured byShikoku Corp.) were uniformly mixed to give a binder.

The ratio by weight of the alkoxy group-containing resol type phenolicresin to the bisphenol F type epoxy resin(phenolic resin:epoxy resin)was 40:60.

A spherical copper powder, prepared by an atomization method, having anaverage particle size of 5.1 μm (product name SFR-Cu, manufactured byNippon Atomized Metal Powders Corporation) was washed with dilutehydrochloric acid and pure water, then subjected to replacement platingusing a plating solution containing 80 g of AgCN and 75 g of NaCN per Lof water so that the amount of silver coating was 3 wt % relative to thespherical copper powder, it was washed with water, and dried to give asilver-plated copper powder (a silver-coated copper powder). During theabove-mentioned drying, the moisture was replaced with ethanol threetimes. In particular, the ethanol used for the third treatment haddissolved therein 0.5 g of stearic acid per kg of the copper powder(corresponding to a coating amount of 0.05 wt % relative to the copperpowder), this ethanol having stearic acid dissolved therein was used forreplacing the moisture contained in the above silver-plated copperpowder, and following this drying was carried out to give a stearicacid-treated silver-plated copper powder.

Subsequently, a 2 L ball mill vessel was charged with 250 g of thestearic acid-treated silver-plated copper powder and 2 kg of zirconiaballs having a diameter of 3 mm, and rotated for 3 hours to give adispersed and surface-smoothed substantially spherical silver-coatedcopper powder having an average aspect ratio of 1.1 and an averageparticle size of 5.1 μm.

The tap density of the substantially spherical silver-coated copperpowder was 65% as a relative value.

A spherical copper powder of the same type as above was subjected to thesame treatments to give a stearic acid-treated silver-plated copperpowder so that the amount of silver coating relative to the sphericalcopper powder (hereinafter, simply called the amount of silver coating)was 12 wt % and the amount of stearic acid coating relative to thespherical copper powder (hereinafter, simply called the amount ofstearic acid coating) was 0.2 wt %.

Subsequently, a 2 L ball mill vessel was charged with 250 g of thestearic acid-treated silver-plated copper powder and 2 kg of zirconiaballs having a diameter of 5 mm, and shaken for 2 hours to give aflat-shaped silver-coated copper powder having an average aspect ratioof 3.1 and an average particle size of 7.3 μm.

The tap density of the flat-shaped silver-coated copper powder was 38%as a relative value.

50 g of the binder so obtained, 436.5 g of the substantially sphericalsilver-coated copper powder, 13.5 g of the flat-shaped silver-coatedcopper powder, and 15 g of ethyl carbitol as a solvent were mixed anddispersed uniformly in a mortar and pestle machine and a three roll millto give an electroconductive paste.

The proportions of the electroconductive powders (the substantiallyspherical silver-coated copper powder and the flat-shaped silver-coatedcopper powder) were such that the substantially spherical silver-coatedcopper powder was 97 wt % and the flat-shaped silver-coated copperpowder was 3 wt %.

The ratio by weight of the binder to the electroconductive powder was10:90 (binder:electroconductive powder).

The electroconductive paste thus obtained was used to print a testpattern 12 on a polyimide film 11 shown in FIG. 3, it was heated in anoven up to 170° C. over 13 minutes, and kept at that temperature for 1hour to give a wiring board.

The wiring board thus obtained was subjected to a pencil scratch test inaccordance with JIS K5401-69 in order to evaluate the curing propertiesof the electroconductive paste, and gave a result of 6H. The sheetresistance of the conductor was 112 mΩ/□.

A test pattern was also printed on the surface of a 1.2 mm thick glasscomposite substrate having a copper foil thereon etched out, and curedby heating under the same conditions as above to give a test substrate.This test substrate was subjected to reliability tests, that is, 4,000hours of a constant temperature and constant humidity test and 3,000cycles of a gas phase cooling and heating test, and the percentagechanges in the circuit resistance were 17.4% and 30.2% respectively. Theconstant temperature and constant humidity test involved storing at 85°C. and 85% RH, and one cycle of the gas phase cooling and heating testcomprised −65° C. for 30 minutes to 125° C. for 30 minutes (the sameapplies below).

The aspect ratio in this example was specifically measured as follows. 8g of a low viscosity epoxy resin main agent (No. 10-8130) and 2 g of acuring agent (No. 10-8132) (manufactured by Buehler) were mixed, 2 g ofthe electroconductive powder was added to this mixture, dispersed well,degassed in vacuum at 30° C. as it was, and then allowed to stand at 30°C. for 10 hours so as to make particles settle and be cured.Subsequently, the cured material thus obtained was sectioned in thevertical direction, the cross section was magnified by 1000 times in anelectron microscope, the ratio of the major axis to the minor axis wasmeasured for 150 particles appearing in the cross section, and theaverage value thereof was defined as the aspect ratio.

Example 2

19 parts by weight of an alkoxy group-containing resol type phenolicresin, 76 parts by weight of a bisphenol F type epoxy resin, and 5 partsby weight of 2-phenyl-4-methylimidazole, all of which were the same asthose used in Example 1, were uniformly mixed to give a binder.

The ratio by weight of the alkoxy group-containing resol type phenolicresin to the bisphenol F type epoxy resin was 20:80 (phenolicresin:epoxy resin).

50 g of the binder so obtained, 427.5 g of the substantially sphericalsilver-coated copper powder obtained in Example 1, 22.5 g of aflat-shaped silver-coated copper powder having an average aspect ratioof 3 and an average particle size of 6.8 μm obtained by the same processas in Example 1 from a stearic acid-treated silver-plated copper powderhaving an amount of silver coating of 5 wt % and an amount of stearicacid coating of 0.5 wt % obtained by the same process as in Example 1,and 10 g of ethyl carbitol as a solvent were mixed and disperseduniformly in a mortar and pestle machine and a three roll mill to givean electroconductive paste.

The proportions of the electroconductive powders (the substantiallyspherical silver-coated copper powder and the flat-shaped silver-coatedcopper powder) were such that the substantially spherical silver-coatedcopper powder was 95 wt % and the flat-shaped silver-coated copperpowder was 5 wt %.

The tap density of the flat-shaped silver-coated copper powder was 41%as a relative value.

Furthermore, the ratio by weight of the binder to the electroconductivepowder was 10:90 (binder:electroconductive powder).

A wiring board was fabricated by the same process as in Example 1, thecharacteristics thereof were evaluated, and it was found that the pencilscratch test for a coating gave a result of 6H, and the sheet resistanceof the conductor was 121 mΩ/□.

A test substrate was fabricated by the same process as in Example 1 andsubjected to reliability tests, that is, 4,000 hours of the constanttemperature and constant humidity test and 3,000 cycles of the gas phasecooling and heating test, and the percentage changes in the circuitresistance were 16.2% and 28.9%, respectively.

Example 3

4.75 parts by weight of an alkoxy group-containing resol type phenolicresin (a trial product of the applicant, number of carbons of the alkoxygroup 4, degree of alkoxylation 65%, weight-average molecular weight20,000), 90.25 parts by weight of the bisphenol F type epoxy resin usedin Example 1, and 5 parts by weight of the 2-phenyl-4-methylimidazoleused in Example 1 were mixed uniformly to give a binder.

The ratio by weight of the alkoxy group-containing resol type phenolicresin to the bisphenol F type epoxy resin was 5:95 (phenolic resin:Ftype epoxy resin).

50 g of the binder so obtained, 436.5 g of a dispersed andsurface-smoothed substantially spherical silver-coated copper powderhaving an average aspect ratio of 1.1 and an average particle size of5.5 μm obtained by the same process as in Example 1 from a stearicacid-treated silver-plated copper powder having an amount of silvercoating of 5 wt % and an amount of stearic acid coating of 0.1 wt %obtained by the same process as in Example 1, 13.5 g of the flat-shapedsilver-coated copper powder obtained in Example 2, and 13 g of ethylcarbitol as a solvent were mixed and dispersed uniformly in a mortar andpestle machine and a three roll mill to give an electroconductive paste.

The proportions of the electroconductive powders (the substantiallyspherical silver-coated copper powder and the flat-shaped silver-coatedcopper powder) were such that the substantially spherical silver-coatedcopper powder was 97 wt % and the flat-shaped silver-coated copperpowder was 3 wt %.

The tap density of the substantially spherical silver-coated copperpowder was 62% as a relative value.

Furthermore, the ratio by weight of the binder to the electroconductivepowder was 10:90 (binder:electroconductive powder).

A wiring board was fabricated by the same process as in Example 1, thecharacteristics thereof were evaluated, and it was found that the pencilscratch test for a coating gave a result of 6H, and the sheet resistanceof the conductor was 156 mΩ/□.

A test substrate was fabricated by the same process as in Example 1 andsubjected to reliability tests, that is, 4,000 hours of the constanttemperature and constant humidity test and 3,000 cycles of the gas phasecooling and heating test, and the percentage changes in the circuitresistance were 15.8% and 40.2%, respectively.

Example 4

4.75 parts by weight of the alkoxy group-containing resol type phenolicresin used in Example 3, 90.25 parts by weight of the bisphenol F typeepoxy resin used in Example 1, and 5 parts by weight of the2-phenyl-4-methylimidazole used in Example 1 were mixed uniformly togive a binder.

The ratio by weight of the alkoxy group-containing resol type phenolicresin to the bisphenol F type epoxy resin was 5:95 (phenolic resin:epoxyresin).

50 g of the binder so obtained, 382.5 g of a dispersed andsurface-smoothed substantially spherical silver-coated copper powderhaving an average aspect ratio of 1.1 and an average particle size of5.5 μm obtained by the same process as in Example 1 from a stearicacid-treated silver-plated copper powder having an amount of silvercoating of 12 wt % and an amount of stearic acid coating of 0.15 wt %obtained by the same process as in Example 1, 67.5 g of a flat-shapedsilver-coated copper powder having an amount of silver coating of 3 wt%, an amount of stearic acid coating of 0.5 wt %, an average aspectratio of 2.2, and an average particle size of 6.2 μm obtained by thesame process as in Example 1, and 16 g of ethyl carbitol as a solventwere mixed and dispersed uniformly in a mortar and pestle machine and athree roll mill to give an electroconductive paste.

The proportions of the electroconductive powders (the substantiallyspherical silver-coated copper powder and the flat-shaped silver-coatedcopper powder) were such that the substantially spherical silver-coatedcopper powder was 85 wt % and the flat-shaped silver-coated copperpowder was 15 wt %.

The tap density of the substantially spherical silver-coated copperpowder was 59% as a relative value, and the tap density of theflat-shaped silver-coated copper powder was 43% as a relative value.

Furthermore, the ratio by weight of the binder to the electroconductivepowder was 10:90 (binder:electroconductive powder).

A wiring board was fabricated by the same process as in Example 1, thecharacteristics thereof were evaluated, and it was found that the pencilscratch test for a coating gave a result of 6H, and the sheet resistanceof the conductor was 124 mΩ/□.

A test substrate was fabricated by the same process as in Example 1 andsubjected to reliability tests, that is, 4,000 hours of the constanttemperature and constant humidity test and 3,000 cycles of the gas phasecooling and heating test, and the percentage changes in the circuitresistance were 9.3% and 26.7%, respectively.

Comparative Example 1

38 parts by weight of an alkoxy group-containing resol type phenolicresin, 57 parts by weight of a bisphenol F type epoxy resin, and 5 partsby weight of 2-phenyl-4-methylimidazole, all of which were the same asthose used in Example 1, were mixed uniformly to give a binder.

The ratio by weight of the alkoxy group-containing resol type phenolicresin to the bisphenol F type epoxy resin was 40:60 (phenolicresin:epoxy resin).

50 g of the binder so obtained, 441 g of a dispersed andsurface-smoothed substantially spherical silver-coated copper powderhaving an average aspect ratio of 1.1 and an average particle size of5.5 μm obtained by the same process as in Example 1 from a stearicacid-treated silver-plated copper powder having an amount of silvercoating of 2 wt % and an amount of stearic acid coating of 0.005 wt %obtained by the same process as in Example 1, 9 g of a flat-shapedsilver-coated copper powder having an average aspect ratio of 4.5 and anaverage particle size of 8.8 μm obtained by the same process as inExample 1 from a stearic acid-treated silver-plated copper powder havingan amount of silver coating of 2 wt % and an amount of stearic acidcoating of 0.05 wt % obtained by the same process as in Example 1, and22 g of ethyl carbitol as a solvent were mixed and dispersed uniformlyin a mortar and pestle machine and a three roll mill to give anelectroconductive paste.

The proportions of the electroconductive powders (the substantiallyspherical silver-coated copper powder and the flat-shaped silver-coatedcopper powder) were such that the substantially spherical silver-coatedcopper powder was 98 wt % and the flat-shaped silver-coated copperpowder was 2 wt %.

The tap density of the substantially spherical silver-coated copperpowder was 64% as a relative value, and the tap density of theflat-shaped silver-coated copper powder was 45% as a relative value.

Furthermore, the ratio by weight of the binder to the electroconductivepowder was 10:90 (binder:electroconductive powder).

A wiring board was fabricated by the same process as in Example 1, thecharacteristics thereof were evaluated, and it was found that the pencilscratch test for a coating gave a result of 6H, but the sheet resistanceof the conductor was a high value of 294 mΩ/□.

A test substrate was fabricated by the same process as in Example 1 andsubjected to reliability tests, that is, 4,000 hours of the constanttemperature and constant humidity test and 3,000 cycles of the gas phasecooling and heating test, and the percentage changes in the circuitresistance were high values of 112.8% and 93.3%, respectively.

Comparative Example 2

4.75 parts by weight of an alkoxy group-containing resol type phenolicresin, 90.25 parts by weight of a bisphenol F type epoxy resin, and 5parts by weight of 2-phenyl-4-methylimidazole, all of which were thesame as those used in Example 1, were mixed uniformly to give a binder.

The ratio by weight of the alkoxy group-containing resol type phenolicresin to the bisphenol F type epoxy resin was 5:95 (phenolic resin:epoxyresin).

50 g of the binder so obtained, 360 g of a dispersed andsurface-smoothed substantially spherical silver-coated copper powderhaving an average aspect ratio of 1.1 and an average particle size of5.5 μm obtained by the same process as in Example 1 from a stearicacid-treated silver-plated copper powder having an amount of silvercoating of 2 wt % and an amount of stearic acid coating of 0.6 wt %obtained by the same process as in Example 1, 90 g of a flat-shapedsilver-coated copper powder having an average aspect ratio of 2.4 and anaverage particle size of 6.3 μm obtained by the same process as inExample 1 from a stearic acid-treated silver-plated copper powder havingan amount of silver coating of 2 wt % and an amount of stearic acidcoating of 2 wt % obtained by the same process as in Example 1, and 21 gof ethyl carbitol as a solvent were mixed and dispersed uniformly in amortar and pestle machine and a three roll mill to give anelectroconductive paste.

The proportions of the electroconductive powders (the substantiallyspherical silver-coated copper powder and the flat-shaped silver-coatedcopper powder) were such that the substantially spherical silver-coatedcopper powder was 80 wt % and the flat-shaped silver-coated copperpowder was 20 wt %.

The tap density of the substantially spherical silver-coated copperpowder was 48% as a relative value, and the tap density of theflat-shaped silver-coated copper powder was 43% as a relative value.

Furthermore, the ratio by weight of the binder to the electroconductivepowder was 10:90 (binder:electroconductive powder).

A wiring board was fabricated by the same process as in Example 1, thecharacteristics thereof were evaluated, and it was found that the pencilscratch test for a coating gave a result of 4H, but the sheet resistanceof the conductor was a high value of 261 mΩ/□.

A test substrate was fabricated by the same process as in Example 1 andsubjected to reliability tests, that is, 4,000 hours of the constanttemperature and constant humidity test and 3,000 cycles of the gas phasecooling and heating test, and the percentage changes in the circuitresistance were high values of 125.1% and 103.8%, respectively.

Comparative Example 3

95 parts by weight of the bisphenol F type epoxy resin used in Example 1and 5 parts by weight of 2-ethyl-4-methylimidazole (product name Curezol2E4MZ, manufactured by Shikoku Corp.) were mixed uniformly to give abinder.

50 g of the binder so obtained, 360 g of a dispersed andsurface-smoothed substantially spherical silver-coated copper powderhaving an average aspect ratio of 1.1 and an average particle size of5.5 μm obtained by the same process as in Example 1 from a stearicacid-treated silver-plated copper powder having an amount of silvercoating of 12 wt % and an amount of stearic acid coating of 0.15 wt %obtained by the same process as in Example 1, 90 g of a flat-shapedsilver-coated copper powder having an average aspect ratio of 6 and anaverage particle size of 7.3 μm obtained by the same process as inExample 1 from a stearic acid-treated silver-plated copper powder havingan amount of silver coating of 12 wt % and an amount of stearic acidcoating of 0.2 wt % obtained by the same process as in Example 1, and 20g of ethyl carbitol as a solvent were mixed and dispersed uniformly in amortar and pestle machine and a three roll mill to give anelectroconductive paste. The shelf life of this electroconductive pastewas 2 days when refrigerated, which was extremely poor compared with 60days for the electroconductive paste obtained in Example 4 whenrefrigerated.

The proportions of the electroconductive powders (the substantiallyspherical silver-coated copper powder and the flat-shaped silver-coatedcopper powder) were such that the substantially spherical silver-coatedcopper powder was 80 wt % and the flat-shaped silver-coated copperpowder was 20 wt %.

The tap density of the substantially spherical silver-coated copperpowder was 58% as a relative value, and the tap density of theflat-shaped silver-coated copper powder was 38% as a relative value.

Furthermore, the ratio by weight of the binder to the electroconductivepowder was 10:90 (binder:electroconductive powder).

A wiring board was fabricated by the same process as in Example 1, thecharacteristics thereof were evaluated, and it was found that the pencilscratch test for a coating gave a result of 6H, but the sheet resistanceof the conductor was a high value of 389 mΩ/□.

A test substrate was fabricated by the same process as in Example 1 andsubjected to reliability tests, that is, 4,000 hours of the constanttemperature and constant humidity test and 3,000 cycles of the gas phasecooling and heating test, and the percentage changes in the circuitresistance were high values of 194% and 216%, respectively.

Example 5

50 parts by weight of a phenoxy resin (product name PKHJ, softeningtemperature 170° C., manufactured by Phenoxy Specialties), 0.4 parts byweight of a titanate coupling agent (product name KR-TTS2, manufacturedby Ajinomoto Co., Inc.), and 75 parts by weight of diethylene glycolmonoethyl ether (product name EtDG, manufactured by Nippon Nyukazai Co.,Ltd.) as a solvent were mixed and dissolved uniformly to give athermoplastic resin solution, which was used as a binder.

125 g of the binder so obtained, 441 g of the substantially sphericalsilver-coated copper powder obtained in Example 1, 9 g of theflat-shaped silver-coated copper powder obtained in Example 1, and 10 gof ethyl carbitol as a solvent were mixed and dispersed uniformly in amortar and pestle machine and a three roll mill to give anelectroconductive paste.

The proportions of the electroconductive powders (the substantiallyspherical silver-coated copper powder and the flat-shaped silver-coatedcopper powder) were such that the substantially spherical silver-coatedcopper powder was 98 wt % and the flat-shaped silver-coated copperpowder was 2 wt %.

The ratio by weight of the binder to the electroconductive powder was10:90 (binder:electroconductive powder).

A wiring board was fabricated by the same process as in Example 1, thecharacteristics thereof were evaluated, and it was found that the pencilscratch test for a coating gave a result of 6H and the sheet resistanceof the conductor was 131 mΩ/□.

When a test substrate was fabricated by the same process as in Example1, the sheet resistance of the test substrate was 85 mΩ/□. When the testsubstrate was subjected to reliability tests, that is 4,000 hours of theconstant temperature and constant humidity test and 3,000 cycles of thegas phase cooling and heating test, the percentage changes in thecircuit resistance were 2.3% and 11.2%, respectively.

A chip resistor was bonded to a copper foil using the electroconductivepaste obtained above, the chip resistor was subsequently detached byheating, and it could easily be detached at a temperature of 180° C.

Example 6

125 g of the binder obtained in Example 5, 405 g of the substantiallyspherical silver-coated copper powder obtained in Example 1, 45 g of theflat-shaped silver-coated copper powder obtained in Example 1, and 10 gof ethyl carbitol as a solvent were mixed and dispersed uniformly in amortar and pestle machine and a three roll mill to give anelectroconductive paste.

The proportions of the electroconductive powders (the substantiallyspherical silver-coated copper powder and the flat-shaped silver-coatedcopper powder) were such that the substantially spherical silver-coatedcopper powder was 90 wt % and the flat-shaped silver-coated copperpowder was 10 wt %.

The ratio by weight of the binder to the electroconductive powder was10:90 (binder:electroconductive powder).

A wiring board was fabricated by the same process as in Example 1, thecharacteristics thereof were evaluated, and it was found that the pencilscratch test for a coating gave a result of 4H and the sheet resistanceof the conductor was 97 mΩ/□.

When a test substrate was fabricated by the same process as in Example1, the sheet resistance of the test substrate was 70 mΩ/□. When the testsubstrate was subjected to reliability tests, that is 4,000 hours of theconstant temperature and constant humidity test and 3,000 cycles of thegas phase cooling and heating test, the percentage changes in thecircuit resistance were 2.1% and 10.5%, respectively.

Example 7

30 parts by weight of a phenoxy resin, 0.5 parts by weight of a titanatecoupling agent, and 85 parts by weight of diethylene glycol monoethylether as a solvent, all of which were the same as those used in Example5, were mixed and dissolved uniformly to give a thermoplastic resinsolution, which was used as a binder.

115 g of the binder so obtained, 460.6 g of the substantially sphericalsilver-coated copper powder obtained in Example 3, 9.4 g of theflat-shaped silver-coated copper powder obtained in Example 1, and 10 gof ethyl carbitol as a solvent were mixed and dispersed uniformly in amortar and pestle machine and a three roll mill to give anelectroconductive paste.

The proportions of the electroconductive powders (the substantiallyspherical silver-coated copper powder and the flat-shaped silver-coatedcopper powder) were such that the substantially spherical silver-coatedcopper powder was 98 wt % and the flat-shaped silver-coated copperpowder was 2 wt %.

The ratio by weight of the binder to the electroconductive powder was6:94 (binder:electroconductive powder).

A wiring board was fabricated by the same process as in Example 1, thecharacteristics thereof were evaluated, and it was found that the pencilscratch test for a coating gave a result of 4H and the sheet resistanceof the conductor was 107 mΩ/□.

When a test substrate was fabricated by the same process as in Example1, the sheet resistance of the test substrate was 63 mΩ/□. When the testsubstrate was subjected to reliability tests, that is 4,000 hours of theconstant temperature and constant humidity test and 3,000 cycles of thegas phase cooling and heating test, the percentage changes in thecircuit resistance were 5.7% and 13.1%, respectively.

Example 8

70 parts by weight of a phenoxy resin, 0.5 parts by weight of a titanatecoupling agent, and 77 parts by weight of diethylene glycol monoethylether as a solvent, all of which were the same as those used in Example5, were mixed and dissolved uniformly to give a thermoplastic resinsolution, which was used as a binder.

147 g of the binder so obtained, 344 g of the substantially sphericalsilver-coated copper powder obtained in Example 4, 86 g of theflat-shaped silver-coated copper powder obtained in Example 4, and 20 gof ethyl carbitol as a solvent were mixed and dispersed uniformly in amortar and pestle machine and a three roll mill to give anelectroconductive paste.

The proportions of the electroconductive powders (the substantiallyspherical silver-coated copper powder and the flat-shaped silver-coatedcopper powder) were such that the substantially spherical silver-coatedcopper powder was 80 wt % and the flat-shaped silver-coated copperpowder was 20 wt %.

The ratio by weight of the binder to the electroconductive powder was14:86 (binder:electroconductive powder).

A wiring board was fabricated by the same process as in Example 1, thecharacteristics thereof were evaluated, and it was found that the pencilscratch test for a coating gave a result of 3H and the sheet resistanceof the conductor was 152 mΩ/□.

When a test substrate was fabricated by the same process as in Example1, the sheet resistance of the test substrate was 103 mΩ/□. When thetest substrate was subjected to reliability tests, that is 4,000 hoursof the constant temperature and constant humidity test and 3,000 cyclesof the gas phase cooling and heating test, the percentage changes in thecircuit resistance were 5.2% and 10.3%, respectively.

Comparative Example 4

125 g of the binder obtained in Example 5, 450 g of the substantiallyspherical silver-coated copper powder obtained in Comparative Example 1,and 10 g of ethyl carbitol as a solvent were mixed and disperseduniformly in a mortar and pestle is machine and a three roll mill togive an electroconductive paste.

The ratio by weight of the binder to the electroconductive powder was10:90 (binder:electroconductive powder).

A wiring board was fabricated by the same process as in Example 1, thecharacteristics thereof were evaluated, and it was found that the pencilscratch test for a coating gave a result of 5H and the sheet resistanceof the conductor was a high value of 198 mΩ/□.

When a test substrate was fabricated by the same process as in Example1, the sheet resistance of the test substrate was a high value of 211mΩ/□. When the test substrate was subjected to reliability tests, thatis 4,000 hours of the constant temperature and constant humidity testand 3,000 cycles of the gas phase cooling and heating test, thepercentage changes in the circuit resistance were high values of 79.5%and 68.7%, respectively.

Comparative Example 5

147 g of the binder obtained in Example 8, 430 g of the substantiallyspherical silver-coated copper powder obtained in Comparative Example 2,and 20 g of ethyl carbitol as a solvent were mixed and disperseduniformly in a mortar and pestle machine and a three roll mill to givean electroconductive paste.

The ratio by weight of the binder to the electroconductive powder was14:86 (binder:the electroconductive powder).

A wiring board was fabricated by the same process as in Example 1, thecharacteristics thereof were evaluated, and it was found that the pencilscratch test for a coating gave a result of 3H and the sheet resistanceof the conductor was a high value of 283 mΩ/□.

When a test substrate was fabricated by the same process as in Example1, the sheet resistance of the test substrate was a high value of 327mΩ/□. When the test substrate was subjected to reliability tests, thatis 4,000 hours of the constant temperature and constant humidity testand 3,000 cycles of the gas phase cooling and heating test, thepercentage changes in the circuit resistance were high values of 119%and 127%, respectively.

Examples 9 to 11 and Comparative Examples 6 and 7

Materials used in Examples 9 to 11 and Comparative Examples 6 and 7 andComparative Examples were prepared by the following methods or wereacquired.

(1) Preparation of Epoxy Resin

10.0 parts by weight of YDF-170 (product name of a bisphenol F typeepoxy resin, epoxy equivalent=170, manufactured by Tohto kasei Co.,Ltd.) and 10.0 parts by weight of YL-980 (product name of a bisphenol Atype epoxy resin, epoxy equivalent=185, manufactured by Japan EpoxyResins Co., Ltd.) were heated at 80° C., and stirred for 1 hour to givea uniform epoxy resin solution.

(2) Preparation of Binder

20.0 parts by weight of the epoxy resin solution prepared in the above(1), 10.0 parts by weight of PP-101 (product name of an alkylphenylglycidyl ether, epoxy equivalent=230, manufactured by Tohto kasei Co.,Ltd.), and 5.0 parts by weight of 2P4MHZ (product name of an imidazolecompound manufactured by Shikoku Corp.) were mixed and kneaded using athree roll mill to give a binder.

The binder prepared in the above (2) and the electroconductive powderused in Example 1 were mixed at the mixing ratio shown in Table 1,kneaded using a three roll mill, and then subjected to degassing at 5Torr or less for 10 minutes to give a paste-like composition.

The characteristics of the compositions of Examples 9 to 11 andComparative Examples 6 and 7 were measured by the following methods. Theresults are summarized in Table 1.

(1) Volume resistivity: the above-mentioned bonding material was formedinto a 1×100×0.25 mm shape and heated at a rate of temperature increaseof 40° C./min and at 200° C. for 5 min to give a test piece. Theresistance R of this test piece was obtained by a four probe method, andthe volume resistivity was calculated from R·L/S using a cross section Sand a length L.

(2) Adhesion: an Sn-plated copper lead frame was coated with about 0.2mg of the bonding material, a 2×2 mm copper chip (thickness about 0.25mm, Ag-plated) was compression-bonded thereto, and the sample was heatedat a rate of temperature increase of 40° C./min form 25° C. to 200° C.and at 200° C. for 5 min for completion of bonding. The adhesive shearstrength (N) of this sample was measured using a bond tester(manufactured by DAGE) with a clearance of 0.05 mm at a shear rate of0.5 mm/sec and 25° C.

(3) Fast curing: when the volume resistively was less than 1×10⁻² an cmand the adhesion was higher than 100 N under heating conditions of arate of temperature increase of 40° C./min and 200° C. for 5 min, it wasevaluated as good, and others were evaluated as poor.

(4) Viscosity: 0.4 mL of an electroconductive paste was placed in an EHDmodel viscometer equipped with a 3 degree cone rotor (manufactured byTokimec Inc.) and the viscosity was measured at 25° C. and 0.5 rpm.

(5) Printing characteristics: when the viscosity was 200 to 400 Pa·s, itwas evaluated as good, and others were evaluated as poor. TABLE 1Electroconductive powder Volume Substantially Flat- resistivity AdhesionFast Viscosity Printing Binder spherical shaped (Ω · cm) (N) curing (Pa· s) characteristics Example 9 A: 25 97 3 1 × 10⁻³ 123 Good 203 GoodExample A: 25 90 10 5 × 10⁻⁴ 109 Good 256 Good 10 Example A: 25 80 20 3× 10⁻⁴ 101 Good 297 Good 11 Comp. A: 25 99 1 5 × 10⁻³ 134 Good 156 PoorEx. 6 Comp. A: 25 75 25 1 × 10⁻⁴ 82 Poor 355 Good Ex. 7

1. An electroconductive paste comprising a binder and anelectroconductive powder containing 80 to 97 wt % of a substantiallyspherical silver-coated copper powder in which the surface of a copperpowder is coated with silver and the surface thereof is further coatedwith 0.02 to 0.5 wt % relative to the copper powder of a fatty acid, and3 to 20 wt % of a flat-shaped silver-coated copper powder in which thesurface of a copper powder is coated with silver and the surface thereofis further coated with 0.02 to 1.2 wt % relative to the copper powder ofa fatty acid.
 2. The electroconductive paste according to claim 1,wherein the substantially spherical silver-coated copper powder has anaverage particle size of 1 to 10 μm and a tap density of 55% to 75% as arelative value to a true density, and the surface thereof is smoothed.3. The electroconductive paste according to claim 1, wherein the bindercomprises as main components an alkoxy group-containing resol typephenolic resin and an epoxy resin together with a curing agent, anadditive, and a solvent therefor.
 4. The electroconductive pasteaccording to claim 3, wherein the epoxy resin has an epoxy equivalent of130 to 330 g/eq.
 5. The electroconductive paste according to claim 3,wherein the alkoxy group-containing resol type phenolic resin has analkoxy group having 1 to 6 carbons.
 6. The electroconductive pasteaccording to claim 3, wherein the alkoxy group-containing resol typephenolic resin has a degree of alkoxylation of 5% to 95%.
 7. Theelectroconductive paste according to claim 3, wherein the alkoxygroup-containing resol type phenolic resin has a weight-averagemolecular weight of 500 to 200,000.
 8. The electroconductive pasteaccording to claim 3, wherein the alkoxy group-containing resol typephenolic resin and the epoxy resin are present at an alkoxygroup-containing resol type phenolic resin:epoxy resin mixing ratio of5:95 to 60:40 as a ratio by weight.
 9. The electroconductive pasteaccording to claim 1, wherein the binder comprises as main components athermoplastic resin together with an additive and a solvent.
 10. Theelectroconductive paste according to claim 9, wherein the thermoplasticresin is a thermoplastic resin having a softening temperature of 90° C.to 240° C.
 11. The electroconductive paste according to claim 9, whereinthe thermoplastic resin is a phenoxy resin.
 12. The electroconductivepaste according to claim 1, wherein the binder comprises as maincomponents an epoxy resin together with a curing agent.